DC amplifiers are well known and widely used. For example, attention is directed to U.S. Pat. No. 3,370,242 dated Feb. 20, 1968, which is assigned to the same assignee as the present application. In a DC amplifier with high gain and high common mode rejection capabilities, zero drift is a problem. The task of eliminating drift has traditionally been accomplished by a mechanical chopper. The chopper is used to produce an amplitude modulated carrier frequency which lies outside the signal bandpass. To date, a chopper stabilized DC amplifier employing solid state circuitry has alluded successful accomplishment. This is due to offset voltages for bipolar transistor choppers and high resistance for field effect transistors.
Additionally, certain considerations must be made when designing a DC amplifier. These include:
1. Voltage offset. PA1 2. Offset current. PA1 3. Common mode rejection. PA1 4. Noise suppression. PA1 5. Drift with temperature.
Each of these considerations has been mastered singly to a high degree, but not in combination. Generally, the requirements of prime importance to the application are maximized and the balance are eliminated from the performance specifications or accepted at a lower criteria level.
Therefore, it is the object of the present invention to provide a solid state amplifier responding to the requirements of all the above enumerated considerations to a high degree of performance excellence.